Sounds to me like your throttle actuator is bad, or you have a vacuum leak that is causing the throttle actuator to "hunt" up and down for the right idle speed.
I had a similar problem to yours, but mine was much worse. My idle oscillated up and down so badly the car was not driveable.
Turns out, I had a vacuum leak at the intake manifold that was causing the problem.
We found this out after trading out the mass air sensor (didn't work), then the throttle actuator (didn't change anything), then to the dealer where they noticed the vacuum leak at the intake manifold seal. Yes, I checked for a vacuum leak at the intake manifold, but couldn't find it.
I kept the old throttle actuator, which is still good. Our cars might take the same actuator.
It costs $961 through Parts Shop.
If yours is bad, I'll sell you mine for $350, which is the price my mechanic will buy it off of me when he needs it for a future customer.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".